It's not just a little screensaver or wallpaper, but it looks like you can scroll round and view the entire map in sections. So far, in the three days the site's been up, its creator, William Bright, has added Hong Kong, New York, Chicago, Boston, Washington and San Franciso and now STOP PRESS he's added London.
On the site William/Little Bill said:
"Next: iPodSubwayMaps goes International!
London, Toronto, Montreal, Ottawa?
ps - the London Underground is a nightmare!"
However, following loads of requests he has done London:
"It's done! Finally! This map is the most requested subway map I've encountered yet. I broke the London Underground map into two sections: a map of downtown London, and a full Underground map. This one took two hours to do."
By downtown he sort of means an elongated version of Zone 1 - or in a separate email I received "one that focuses specifically on the craziness that is downtown".